In recent years, for example, techniques such as cross traffic alert (CTA) and lane change aid (LCA) have been developed as a technique for supporting driving of cars. In these systems, a technique of detecting obstacles in surroundings areas using a radar is employed. The radar radiates a radio wave in a predetermined direction, and when the radio wave is reflected by an obstacle, the obstacle is detected by receiving and processing the reflected wave. To this end, the radar includes a transmitting antenna that radiates a radio wave and a receiving antenna that receives a reflected wave.
For example, as illustrated in FIG. 2, in-vehicle radar antennas used for detecting an obstacle are installed at four corners of a vehicle 10, and each antenna is installed to detect an obstacle in a predetermined range around the vehicle. In order to detect an obstacle around the vehicle with a high degree of accuracy through the radar, it is desirable that the antenna installed on the radar have a wide directivity in a horizontal direction and a narrow directivity in a vertical direction in order to suppress unnecessary radio waves. For example, an antenna 91 installed on a front left corner of the vehicle 10 illustrated in FIG. 2 is installed such that an antenna surface is erected vertically, and a vertical direction (radiation direction) of the antenna surface has an angle of 45° from a vehicle front side on a horizontal plane and an angle of 45° from a vehicle left side as indicated by an arrow in FIG. 2).
The in-vehicle radar antenna 91 illustrated in FIG. 2 has a directivity of a range of about ±60° on the horizontal plane in order to detect obstacles around the vehicle, but it is desirable that the in-vehicle radar antenna 91 has a directivity in which a gain is high, particularly, in a vehicle front direction of −45° and a vehicle left direction of +45°. It is desirable that the antennas at the other three corners have a similar directivity.
In order to control the directivity of the antenna, a method of using a shape of a radome covering the antenna is known. For example, Patent Document 1 discloses a microwave antenna in which a directivity of a wide angle is obtained by installing an opening or a thin surface portion having a predetermined shape in a dielectric covering a front surface of an antenna opening and forming an outer corner of an antenna opening end side edge portion on a curved surface. Further, it is stated that beam control can be performed by changing conditions such as a shape, an arrangement, and the like of the dielectric.
Further, Patent Document 2 discloses a wide coverage radar apparatus in which a radome having a transmitting antenna and a receiving antenna accommodated therein reduces influence on a radiation characteristic of each antenna. It is stated that it is possible to reduce the influence of the radome by adjusting a thickness and a radius of curvature of a corner portion of the radome. Further, it is stated that it is possible to implement a wide angle of directivity of each antenna and improving isolation between both antennas by adjusting the thickness of the radome between the transmitting antenna and the receiving antenna. For example, it is stated that it is possible to improve a characteristic by reducing the thickness of the radome.